Many different types of medical devices can be implanted within patients to provide medical therapy. One common type of implanted medical device is a cardiac rhythm management device, such as a pacemaker or an implantable cardioverter defibrillator (ICD). Cardiac rhythm management devices can be used to provide medical therapy to patients who have a disorder related to cardiac rhythm. For example, a pacemaker may be used to treat a patient exhibiting bradycardia. Pacemakers are generally configured to provide a pacing pulse as necessary to control the heart rate.
Magnetic resonance imaging (MRI) is a technique for visualizing body tissues of a patient for purposes of medical diagnosis and therapy. MRI relies on subjecting the body tissue of interest to a very strong uniform magnetic field, up to about 30,000 gauss, as well as a moderate strength but variable magnetic field of around 200 gauss. In the presence of these uniform and gradient magnetic fields, a radio frequency (RF) pulse is transmitted from a coil to the body tissue. Hydrogen atoms within the body tissue have a magnetic moment and tend to line up with the direction of the applied magnetic fields. Some of these hydrogen atoms will align facing one direction and others will align facing an opposite direction, such that most of the hydrogen atoms facing in alternating directions will tend to cancel each other out. However, a small percentage (but a significant absolute number) of hydrogen atoms will be unbalanced, or not cancelled out. The applied RF pulse tends to cause the unbalanced hydrogen protons to spin, or resonate, in a particular direction and at a particular frequency. When this RF pulse is turned off, the spinning hydrogen protons revert to their earlier, aligned position, and release their excess energy. The RF coil of the MRI machine is capable of detecting this released energy and transmitting a corresponding signal to a processor that in turn transforms the signal into an image of the body tissue. Because different tissues have different characteristic responses to the application of the RF pulse in the presence of the magnetic fields, these differences can be utilized to prepare an image showing areas of contrasting tissue types.
MRI techniques have proven to be very effective at diagnosing certain medical conditions and therefore allowing for patients to receive timely, appropriate medical therapy. However, in many cases patients having an implanted medical device are contraindicated for MRI, and therefore may be unable to benefit from the full scope of diagnostic techniques available. One problem is that the MRI can induce a current within elements of the implanted medical device. A principle known as Faraday's law states that any change in a magnetic field around a conductive loop will cause a voltage to be induced in the conductive loop, and consequently, cause a current to flow in the conductive loop. In the case of a patient undergoing an MRI procedure, the time-varying magnetic field gradients of the MRI machine create the required changing magnetic field and elements of the implanted cardiac rhythm management device form a conductive loop, resulting in a current flow. Unfortunately, this current flow can interfere with the proper functioning of the medical device.
For at least these reasons, implantable medical devices are needed that can be used during MRI examinations.